1. Field of the Invention
The present invention relates to a process of quantizing tone-level information of an image.
2. Description of the Related Art
Image data having multi-tonal levels (to be referred to as multiple tone-level image data hereinafter) read by an image input apparatus such as a scanner, and multiple tone-level graphic image data created/edited by a computer, are reproduced as images by, for example, a display, a printer, a facsimile machine, or a digital copying machine. At this time, to decrease the amount of image data so as to store and transfer the multiple tone-level image data, a tone-level conversion process for decreasing the number of tonal levels of each pixel is performed. When tone-level conversion is performed in accordance with the tone-level representation capacity per pixel of an image output apparatus such as a printer or an offset printing press, the tone-level conversion process is necessary as well.
Various methods are available for the tone-level conversion process for decreasing the number of tonal levels. For example, an error diffusion method and a minimized average error method that is nearly equivalent to the former method are widely used to attain a satisfactory image quality.
However, the error diffusion method and the minimized average error method pose the following problems.
The first problem is a delay of pixel generation. In other words, generation of black pixels in a density region (to be referred to as a rise region hereinafter) in which the density increases from low densities to middle and high densities delays, and generation of white pixels in a density region (to be referred to as a fall region hereinafter) in which the density decreases from high densities to middle and low densities similarly delays. Note that the delay of pixel generation means that upon examining the pixel values in the order of rasterization, a transition from a low-density region to a middle- and high-density region has already been made in multiple tone-level image data, while that from a low-density region to a middle- and high-density region has not yet been made in corresponding image data having undergone tone-level conversion. Again, the delay of pixel generation similarly means that a transition from a high-density region to a middle- and low-density region has already been made in multiple tone-level image data, while that from a high-density region to a middle- and low-density region has not yet been made in corresponding image data having undergone tone-level conversion.
The second problem is associated with the reproducibility of a fine line. In other words, it is often the case that a fine line has so small a difference in density from the background region that it is reproduced upon being cut off halfway (to be referred to as discontinuation hereinafter) or it is not reproduced at all (to be referred to as disappearance hereinafter).
These problems occur because a diffusing error adversely affects adjacent pixels. In order to solve these problems, Japanese Patent Laid-Open No. 2002-374412 sets a binarization threshold value T in accordance with:T(x,y)={I(x,y)×(K−1)+128}/K  (1)where I(x, y) is the pixel value of an original image at the pixel position (x, y), and K is an arbitrary natural number equal to or more than two.
The above-mentioned method in which the binarization threshold value T(x, y) is uniquely set from the original image data I(x, y) produces a certain effect of preventing pixel generation from delaying, but still cannot achieve a sufficient reproducibility of a fine line, resulting in discontinuation or disappearance.